JP2013539061A - Bi-directional cylindrically symmetric polarization converter and Cartesian-cylindrical polarization conversion method - Google Patents

Abstract

The present invention relates to a bi-directional Cartesian-cylindrical converter (7) for converting the spatial distribution of polarization states, which converter receives a linearly polarized, spatially uniform polarization distribution beam and converts it. A radial polarization converter (3) for converting into a beam having a radial or azimuth distribution of the polarization state with respect to the axis of symmetry, and optical means (6) for compensating for the phase shift caused by said radial polarization converter (3) The optical compensation means (6) is capable of introducing a spatially uniform phase shift so as to compensate for the phase shift introduced by the radial polarization converter (3).
[Selection] Figure 5

Description

  The present invention relates to an optical polarization converter system. More particularly, the present invention is a bi-directional optical system for converting a spatially uniform polarized beam into a beam having a cylindrically symmetric polarization state distribution with respect to the optical axis in a first direction of propagation. Both of the beams capable of converting a beam having a cylindrically symmetric polarization state distribution generalized with respect to the optical axis into a spatially uniform polarized beam in a direction of propagation opposite to the first direction. TECHNICAL FIELD The Cartesian-cylindrical polarization converter of the present invention is preferably achromatic.

  A large number of optical measurement devices, such as polarimeters, ellipsometers, etc., use the specific property of light that is polarized in the light-matter interaction. Scientific instrumentation particularly uses polarized light emitted by a laser source, more often linearly polarized light. Imaging techniques also use polarized light properties to improve image contrast. Therefore, a phase contrast microscope or interference contrast microscope incorporates a birefringent element and subtracts the contribution of the scene to improve the contrast of the image so that the light beam is split into two beams that have polarization states that are perpendicular to each other. Split and recombine them.

  More often, a spatially homogeneously polarized light beam is used, which is either directly generated by the light source (eg, a laser) or unpolarized light source and one or several optics It is generated by a combination of components (polarizer, wave plate, polarization rotator ...). The polarization state for a beam with a spatially homogeneous polarization state distribution in that case is a vector basis defining two Cartesian axes (x, y) in the wave plane perpendicular to the optical axis of the beam. It can be described by its coordinates (see FIG. 1B).

  However, in much more applications it is desirable to use spatially inhomogeneous polarized light. In this case, the polarization state of the beam is more often a heterogeneous polarization state distribution with rotational symmetry (eg radial or azimuth polarization), for which a polar coordinate basis (r, φ) Is more suitable (FIG. 1B). In fact, it has been demonstrated that the focus setting of a radially polarized beam allows reaching a beam size below the critical diameter of the unpolarized beam focus setting, which is a major advance in microlithography, for example. Configure.

  The interest for beams having a radial or azimuth polarization state distribution is accompanied by a need for components that allow conversion from a homogeneous polarization state distribution to an inhomogeneous polarization state distribution.

  A variety of components have been developed that convert a spatially uniform linearly polarized beam into a beam with either a radial or azimuth polarization distribution. The available components are based on a variety of technologies such as:

  The first category comprises components formed from fan-shaped wave plate assemblies whose axes are angularly distributed so as to locally modify the polarization of the incident beam; For example, U.S. Patent No. 6,057,089 describes a component that has eight distinct sectoral sector areas. However, this type of component does not make it possible to obtain a continuous spatial polarization state distribution.

  The second category includes optical components based on combinations of diffraction grating patterns as described in the publications of US Pat.

  -Finally, mention is made of components based on liquid crystals. The non-patent document 2 describes a radial and azimuth polarization converter based on a liquid crystal cell, which uses a linearly polarized spatially uniform light beam and the direction of incident linearly polarized light in a liquid crystal cell. Can be converted into either a radial or azimuth polarized light beam depending on whether it is parallel or perpendicular to the alignment axis. The contents of the non-patent document 3 describe components having the same characteristics.

  The contents of Stallder and Schadt (Non-Patent Document 2) describe in particular a liquid crystal cell comprising two main liquid crystal layers, in which the liquid crystals of the first layer are mutually connected. The liquid crystals of the second layer are arranged in concentric circles (see FIG. 1A). The intermediate liquid crystal located between the layers on both ends gradually rotates between the two final states (see FIG. 1B). In the contents of the publication by Ko et al. (Non-patent Document 3), the second liquid crystal layer is directed radially.

  Liquid crystal components provide the advantage that they can be made rather easily in a variety of configurations. Generally speaking, optical liquid crystal components are made by “rubbing” and photo-alignment techniques.

  However, various existing polarization converters perform polarization conversion for specific conditions where the incident beam is linearly polarized and aligned either parallel or perpendicular to the natural axis of the converter. It is important to note here. A linear polarization state parallel to either one of the two natural axes of the transducer actually gives a polarization distribution of either radial or azimuth after conversion to the polar base.

  The use of a beam that is spatially inhomogeneous, in particular a beam with a polarization distribution that is spatially distributed according to cylindrical symmetry, is that an optical system in which the polarized beam is rotationally symmetric in optical properties (eg optical lenses, microscopes) Of particular interest when coupled to an objective mirror or optical fiber. In fact, in the case where a plane wave beam of homogeneously (eg, linearly) polarized light illuminates the ideal lens 4 according to its optical axis 10, the image points formed in the imaging focal plane are geometrically In a typical optical system, it can be described by a set of rays with incident angle θ and azimuth φ of the incident plane (see FIG. 2). However, due to the rotational symmetry of the lens 4 with respect to the optical axis 10, the polarization state distribution of the beam is altered both by the passage of the lens 4 and by the reflection at the sample 5, so that the polarization state of the incident and reflected beam is the lens 4. It depends on the incident plane from the moment of passing. In contrast, a beam having either a radial or azimuth polarization distribution centered on the optical axis 10 of the focal lens 4 does not disturb its polarization vector distribution by coupling to the focal lens 4. In cylindrical geometry, each azimuth plane is exactly the same across the lens. The polarization state distribution after passing through the lens 4 remains as a radial polarization distribution or an azimuth polarization distribution, respectively. The polarization state distribution of either radial or azimuth therefore has the advantage that it is not affected by the focus lens 4.

  However, the polarization converter systems shown so far only allow acquisition of a specific spatial polarization distribution, operate only in specific use conditions and are radial from a uniform linearly polarized beam directed along the converter axis. It is possible to obtain a polarization distribution of either azimuth or azimuth.

  In contrast, from a beam having an elliptical polarization state with a uniform spatial distribution, prior art radial polarization converters generally generate a beam with a cylindrically symmetric polarization state distribution, which polarization state is already in the initial elliptical state. Not in state. In addition, the cylindrically symmetric polarization state obtained from the output also has a large dependence on wavelength. There are no components that allow operations for an overall polarization state basis, basis change, or bijection to do Cartesian coordinates to polar coordinates or vice versa.

  In the technical field of non-uniformly distributed polarized beams, in particular, polarized beams (cylindrical vector beams or CV beams) having a cylindrically symmetric type polarization state distribution are discussed herein. Radial or azimuth polarized beams are both specific cases of cylindrically symmetric beams, where the polarization state is linear and the polarization axis is either in the radial or azimuth direction relative to the optical axis of the beam. Distributed. There are other beams having a cylindrically symmetric polarization state distribution. First, there is a linearly polarized beam whose polarization axis has a constant angle with respect to the local radial axis. Such a beam is obtained, for example, by a linear combination without phase shift between the two components p and s. A cylindrically symmetric elliptically polarized beam is also included, which includes an elliptical polarization state where the polarization distribution is spatially uniform, whose elliptical axis is tilted at a constant angle with respect to the local radial angle.

  A generalized cylindrically symmetric polarized beam is a beam having a cylindrically symmetric polarization state distribution that is not limited to either radial polarization or azimuth polarization.

  There is no polarization converter capable of generating any polarization state distribution (CV beam) with generalized cylindrical symmetry from a beam having a uniform polarization state distribution. Conversely, there is no polarization converter capable of receiving a generalized cylindrically symmetric polarization beam and converting it to a spatially uniform polarization beam.

US Patent Application Publication No. 2007/0115551 US Pat. No. 7,570,427 B1

M.M. M. Lerman and U.S. “Generation of a Radially Polarized Beam Spacing” by L. Levy (Generation of a Radially Polarized Light Beam Using Space-Variant Subwavelength Gratings at 1064 nm variant subwavelengths at 1064 nm) "Optics Letters 33, No. 23, pp 2783-2784, 2008. M.M. Staller and M.S. “Mr. Schadt”, “Linearly Polarized Light with Axial Symmetrically Generated by Liquid-Crystal Polarization Converters” Optics Letters, Vol. 21, No. 23, pp 1948-1950, 1996. S-W Ko (S-W Ko), C.I. C. Ting, A.C. A. Fuh and T.W. "Polarization converters based on axially twisted nematic liquid optic" by T.Lin. "Polarization converters based on axially symmetrical twisted nematic liquid crystal" ) 18, p3601, 2010

  The present invention aims to remedy these drawbacks and, more particularly, relates to a bidirectional Cartesian to cylindrical spatial polarization state distribution converter, said Cartesian to cylindrical converter being in a first direction of propagation, The ability to convert a beam having a uniform spatial polarization state distribution into a beam having a generalized cylindrically symmetric spatial polarization state distribution, and in a second direction of propagation, a generalized cylindrically symmetric spatial polarization state Ability to convert a beam having a distribution into a beam having a uniform spatial polarization state distribution.

According to the present invention, a Cartesian to cylindrical converter is
-A radial and azimuth polarization converter with an axis of symmetry capable of receiving a linearly polarized beam with a spatially uniform polarization distribution and converting it into a beam with a radial or azimuth symmetric polarization distribution Radial and azimuth polarization converters;
An optical means for compensating for the phase difference caused by the radial and azimuth polarization converter, so as to compensate for the magnitude of the phase shift χ introduced by the radial and azimuth polarization converter, And an optical compensation means capable of introducing a spatially uniform phase shift having a magnitude equal to.

According to a preferred embodiment of the present invention,
The radial and azimuth polarization converter comprises a first Cartesian plane of symmetry and a second cylindrical plane of symmetry, and the optical compensation means having the natural axis of polarization is on the side of the Cartesian plane of the radial and azimuth polarization converter And the natural axis of the optical compensation means is aligned with the natural axis of the Cartesian surface,
The radial and azimuth polarization converter comprises a liquid crystal converter having a first surface with a rectilinear alignment of the liquid crystal and a second surface with a concentric liquid crystal alignment;
The optical compensation means comprises a birefringent plate with two natural axes, the birefringent plate being able to introduce a phase difference between linearly polarized light oriented according to its natural axis, A birefringent plate is disposed on the first Cartesian side of the radial and azimuth transducer.

According to various aspects of embodiments of the present invention,
The birefringent plate has a thickness capable of compensating for the phase difference of the radial and azimuth polarization converter over said wavelength range, so that this Cartesian-cylindrical polarization converter is achromatic over the wavelength range;
The optical compensation means comprises a liquid crystal cell;

The invention also relates to an optical system comprising a Cartesian to cylindrical polarization converter according to an embodiment of the invention, according to various aspects of the invention,
-Comprising at least one polarizing beam splitting filter;
At least one optical lens centered on the axis of symmetry of the radial polarization converter, comprising a lens arranged on the cylindrical surface side of the radial converter;

  The present invention also provides a Cartesian-cylinder polarization state converter microscope objective comprising a Cartesian-cylinder polarization converter according to one of the preceding embodiments and a mechanical fixture adapted to be fixed on the microscope. Also related to mirrors.

  Finally, the present invention provides a radial and azimuth polarization converter, and optical means for compensating for the phase difference between the radial and azimuth polarization caused by the radial and azimuth polarization converter, the radial and azimuth polarization converter comprising: Using optical compensation means capable of introducing a spatially uniform phase shift with a magnitude equal to -χ to compensate for the magnitude of the phase shift χ introduced by the azimuth polarization converter, In a first direction of propagation, a beam having a uniform spatial polarization state distribution is converted into a beam having a generalized cylindrically symmetric spatial distribution of the same polarization state and / or a second direction of propagation Transforms a beam with a generalized cylindrically symmetric spatial polarization state distribution into a beam with a uniform spatial distribution with the same polarization state. It also relates to a method of to-cylinder polarization conversion.

  The invention will find particularly advantageous applications in the field of spatially resolved ellipsometry, micro ellipsometry, ellipsometry contrast microscopy, field contrast microscopy, or polarization microscopy.

  The present invention also relates to features that will become apparent from the following description, which will be considered either alone or in any combination thereof that is technically possible.

  This description is given by way of non-limiting example and, together with reference to the accompanying drawings listed below, allows a better understanding of how the present invention can be implemented.

1 is an exploded perspective view of a radial and azimuth transducer according to the prior art. FIG. It is explanatory drawing which showed the expression of the polarization state base in a Cartesian and cylindrical system. It is the perspective view which showed schematically the outline | summary of the principle of propagation | transmission of the polarized beam after the double passage of an objective mirror and the reflection on a sample. It is the top view which showed schematically the effect of the radial and azimuth polarization converter with respect to various polarization states. FIG. 6 is a plan view schematically showing the effect of the polarization converter according to the present invention on various polarization states. 1 is a perspective view schematically illustrating an optical polarization converter system according to an embodiment of the present invention. FIG. 1 is a cross-sectional view schematically showing an objective mirror of a microscope integrated with a polarization converter system according to an embodiment of the present invention. FIG. 6 is a graph of measured values showing phase shift introduced by radial and azimuth converters and a graph of phase shift of a birefringence compensator. 3 is a graph showing residual phase shift as a function of wavelength.

For example, the operation of the radial and azimuth polarization converter shown schematically in FIG. 1A, according to the principles set forth by Stallder and Schadt, is first analyzed. Such a radial polarization converter 3 has a plane 3a in which the liquid crystal is aligned according to an axis transverse to the optical axis and a plane 3b in which the liquid crystal is aligned according to a concentric circle centered on the optical axis. A cell with a layer of liquid crystal is used. However, the liquid crystal cell of the radial converter 3 creates birefringence by introducing an optical path difference between two polarizations aligned according to the natural axis of the converter (usually the refraction axis n _o and the extraordinary refraction axis n _e ). The value of the optical path difference, also called phase difference, which is the product of the thickness d of the difference Δn = n _e -n _o and passes the liquid crystal layer. This optical path difference introduces a phase shift χ between the two polarizations aligned according to the natural axis of the transducer, which itself depends on the wavelength λ. The phase shift χ is given by the following formal formula:

  FIG. 1B shows a representation of the polarization state basis in a Cartesian and cylindrical system. This represents a representation according to their distribution of crystal orientation between the two end layers in this system. The effect of passing through the radial polarization converter 3 can be expressed in the form of a Jones transfer matrix through components as described in the Stallder and Schadt announcement. By representing the initial and final polarization states in the basis (x, y), this Jones matrix can be written as:

  By expressing the initial polarization state in the basis (x, y) and the final polarization state in the cylindrical basis, this Jones matrix can be written as:

  Therefore, a phase shift χ due to the passage of the component appears.

  FIG. 3 schematically shows the effect of a radial and azimuth polarization converter on various polarization states in the direction of propagation from the “Cartesian” surface 3a to the “cylindrical” surface 3b.

  FIG. 3A shows the effect of passing through a radial converter for an incident beam that is linearly polarized and aligned parallel to the natural axis of the input surface 3a, which has a uniform spatial polarization state distribution. (Represented schematically on the left side of the transducer by a vertical arrow). At the output of the radial converter 3 on the “cylindrical” surface side, the linearly polarized and spatial polarization state distribution of FIG. 3Z is obtained (represented schematically on the right side of the converter). It is done.

  FIG. 3B shows the transformation of an incident beam that is linearly polarized and aligned perpendicular to the natural axis of the input surface 3a, which has a uniform spatial polarization state distribution (horizontal arrows). Is schematically represented on the left side of the transducer). At the output of the radial converter 3 on the “cylindrical” plane side, the linearly polarized beam shown in FIG. 3Y and having a spatial polarization state distribution in the azimuth direction is obtained.

  FIG. 3C schematically illustrates the conversion of an incident beam that is linearly polarized and tilted with respect to the natural axis of the input face 3a of the radial converter (ie parallel (FIG. 3A) and perpendicular (FIG. 3B)). Linear combination of polarization). At the output of the radial converter 3 on the “cylindrical” plane side, an elliptically polarized beam (due to the phase shift caused by the radial converter 3), not linearly polarized light, of FIG. 3X is obtained. The distribution of the beam of FIG. 3C at the output of the radial converter has rotational symmetry. It is therefore a polarized beam (CV beam) with cylindrical rotational symmetry.

  FIG. 3D schematically shows the conversion of the circularly polarized incident beam. At the output of the radial converter 3 on the “cylindrical” plane side, the elliptically polarized beam resulting from the phase shift caused by the radial converter 3 is obtained, not the circularly polarized light of FIG. 3W.

  The first observation in the present invention is that the radial and azimuth transducers maintain the polarization state only for linear polarization states that are either parallel or perpendicular to the natural axis of the radial transducer 3.

  The present invention proposes an optical system that allows the switching from a polarization state in a Cartesian system to the same polarization state in a polar coordinate system to be fully operational whatever the polarization state of the incident beam.

  According to a first embodiment, the optical polarization converter system 7 of the present invention comprises a radial polarization converter 3 combined with compensation means, the compensation means introduced by the radial polarization converter 3. A phase shift χ ′ substantially opposite to χ is introduced, in other words χ + χ′≈0.

  By expressing the initial and final polarization states in the basis (x, y), the Jones matrix of the converter according to the invention is described as follows:

  By expressing the initial polarization state in the basis (x, y) and the final polarization state in the cylindrical basis, this Jones matrix can be written as:

  FIG. 5 illustrates a polarization converter system according to a preferred embodiment of the present invention. The optical polarization converter system 7 comprises means for compensating for the phase shift and a radial polarization converter 3 centered on the optical axis 10. The phase shift compensation means comprises a wave plate 6 which is arranged in front of the radial polarization converter 3 on the side of the uniform polarization beam 8 and on the “Cartesian” surface 3a side of the radial and azimuth converter 3. Yes. The wave plate 6 may be a fixed wave plate, or may be a so-called multi-order wave plate in which the phase difference Δn × d is appropriately calculated by selection of materials and thickness necessary for polarization conversion, or The liquid crystal layer may be a liquid crystal layer that is exactly the same as the radial polarization converter 3 and in which the liquid crystal of the wave plate is disposed between two linearly oriented surfaces. The orientation of the liquid crystal layer of the wave plate 6 is then selected to be perpendicular to that of the surface 3a of the radial converter 3, and the liquid crystals for themselves are parallel to each other (see FIG. 5). In this method, the abnormalities and the normal axis of the liquid crystal cell are replaced in the subsequent liquid crystal cell.

  Known radial and azimuth liquid crystal converters may be equipped with a wave plate covering half of the converter. This compensator generally has a phase difference that can be adjusted such that when the operating wavelength selected by the user is λ, its value can be fixed at λ / 2. Such a wave plate exhibits the function of changing the direction of the polarization state without changing its direction. Indeed, it is observed that the radial or azimuth polarization state radiation produced at the output is constant over each half of the components, but each of those halves has a phase state that differs by an angle π. Such a “semi-cylindrical” phase distribution can be detrimental in certain applications. In order to obtain a cylindrically symmetric phase distribution, it is known to add a compensation wave plate having a phase difference of λ / 2. Such a compensator (referred to as a λ / 2 plate) has the effect of compensating for the phase difference introduced by the radial converter between the various components of polarized light passing through it within the same point. Never show. Here, on the one hand, the waveplate intervening in the interpolation covers the entire surface, not half of the radial and azimuth transducers, and on the other hand, in the example described here it is much more than λ / 2 I want to emphasize that it has a high (and even more than 5λ / 2) phase difference.

  On the contrary, according to the invention, the phase difference introduced by the plate 6 is equal to the optical path between the slow axis of the plate and its fast axis equal to the phase difference Δn × d added by the radial and azimuth converters. Selected to introduce the difference. In addition, the orientation of the plate 6 is selected so that its slow axis coincides with the fast axis of the radial and azimuth converter, so that the rays passing through the wave plate 6 and the radial and azimuth converter 3 The optical path difference between the polarizations will compensate each other. Thus, passing through the wave plate 6 introduces a phase shift that compensates for the phase shift introduced by the radial polarization converter for each of the wavelengths.

  FIG. 4 schematically shows the effect of the polarization converter according to the invention on various polarization states in the direction of propagation from a Cartesian system to a cylindrical system.

  FIG. 4A shows the effect of passing through the transducer 7 of the present invention for an incident beam that is linearly polarized and aligned parallel to the natural axis of the input Cartesian surface of the transducer 7. Having a spatial distribution of spatial polarization (represented schematically on the left side of the transducer by a vertical arrow). At the output of the converter 7 on the cylindrical system side, a linearly polarized beam with a spatial polarization state distribution (shown schematically on the right side of the converter) in FIG. 4Z is obtained.

  FIG. 4B shows the transformation of an incident beam that is linearly polarized and aligned perpendicular to the natural axis of the input Cartesian surface of the transducer 7, which has a uniform spatial polarization state distribution. (Schematically represented on the left side of the transducer by a horizontal arrow). At the output of the converter 7 on the cylindrical system side, the beam of FIG. 4Y that is linearly polarized and whose spatial polarization state distribution is in the azimuth direction is obtained.

  FIG. 4C schematically shows the transformation of an incident beam that is linearly polarized and tilted at 45 degrees with respect to the natural axis of the input Cartesian plane of the transducer 7 (ie parallel (FIG. 4A) and perpendicular ( FIG. 4B) Linear combination of polarization). At the output of the transducer 3 on the “cylindrical” system side, the beam with the linear polarization state of FIG. 4X is obtained, and the polarization state distribution is a straight line inclined at 45 degrees with respect to the local radial axis. Polarized light. The spatial polarization state distribution at the output of the transducer 7 of the beam of FIG. 4C has rotational symmetry. This is actually a polarized beam (CV beam) with cylindrical rotational symmetry.

  FIG. 4D schematically illustrates conversion of a circularly polarized incident beam (ie, linear combination due to phase shift between parallel linear polarization state and vertical linear polarization state). At the output of the transducer 7 on the cylindrical system side, a circularly polarized beam having rotational symmetry as shown in FIG. 4W is obtained. Therefore, this is actually a polarized beam (CV beam) with cylindrical rotational symmetry.

  FIG. 4E schematically illustrates the transformation of an incident beam having an elliptical polarization state in which the major axis of the ellipse is aligned according to the natural axis of the input Cartesian surface of the transducer 7. At the output of the converter 7 on the cylindrical system side, the elliptically polarized beam of FIG. 4V is obtained. The spatial distribution of this polarization state is such that the principal axes of the ellipses are locally aligned according to the radial direction. Since the passage through the transducer 7 does not cause a phase shift, the elliptical polarization state is not changed, only the spatial distribution, ie only the direction of the ellipse, is changed. Again, the beam of FIG. 4V (CV beam) having a polarization state distribution which is cylindrically symmetric with respect to the optical axis is obtained.

  FIG. 4F schematically illustrates the conversion of an incident beam having an elliptical polarization state in which the major axis of the ellipse is transverse to the natural axis of the input Cartesian surface of the transducer 7. At the output of the converter 7 on the cylindrical system side, the elliptically polarized beam shown in FIG. 4U is obtained. The spatial distribution of the polarization state is such that the main axis of the ellipse is locally aligned according to the azimuth direction. Since the passage through the transducer 7 does not cause a phase shift, the elliptical polarization state is not changed, only the spatial distribution, ie only the direction of the ellipse, is changed. This also gives the beam of FIG. 4U (CV beam) having a cylindrically symmetric polarization state distribution with respect to the optical axis.

  Finally, FIG. 4G schematically illustrates the conversion of an incident beam having an elliptical polarization state in which the major axis of the ellipse is tilted at 45 degrees with respect to the natural axis of the input Cartesian surface of the transducer 7. At the output of the converter 7 on the cylindrical system side, the elliptically polarized beam shown in FIG. 4T is obtained. The spatial distribution of the polarization state is such that the main axis of the ellipse is locally inclined at 45 degrees with respect to the radial direction. Since the passage through the transducer 7 does not cause a phase shift, the elliptical polarization state is not changed, only the spatial distribution, ie only the direction of the ellipse, is changed. This also yields the beam of FIG. 4T (CV beam) having a polarization state distribution that is cylindrically symmetric with respect to the optical axis.

  Thus, the transducer according to the invention tilts the polarization state of the beam for a linear, circular or elliptical polarization state with respect to the natural axis of the Cartesian plane of the transducer 7 It is observed to maintain even in cases. It is also observed that the transducer system of the present invention allows the generation of a beam with a cylindrically symmetric spatial polarization state distribution whatever the polarization state of the incident beam.

  More generally speaking, this polarization converter allows switching from a beam having some uniform polarization state to a beam with a cylindrically symmetric polarization state distribution with respect to the optical axis 10.

  Conveniently, the wave plate 6 consists of a layer of liquid crystal parallel to each other but oriented perpendicular to the first layer of the radial polarization converter, which wave plate is a Cartesian of the radial and azimuth converter. Arranged on the side of the surface, which makes it possible to obtain a completely achromatic correction of the phase shift.

  FIG. 5 shows the bidirectional operation of the Cartesian-to-cylinder converter. In the first direction of propagation, a beam 8 having a spatially uniform polarization state distribution is directed onto the wave plate 6 and then directed onto the Cartesian plane 3 a of the radial converter 3. After passing through the radial polarization converter 3, the beam 8 exits in the form of a beam having a cylindrically symmetric polarization state with respect to the optical axis 10. In the opposite propagation direction, a beam 9 having a cylindrically symmetric polarization state distribution with respect to the optical axis 10 is directed onto the rotationally symmetric surface 3 b of the radial converter 3. After passing through the radial polarization converter 3 and the wave plate 6, the beam 9 has a spatially uniform polarization state because the wave plate 6 accurately compensates for the phase shift introduced by the radial converter 3. Go out in the form of a beam.

  By reciprocity, the polarization converter 7 of the present invention converts a beam having a cylindrically symmetric polarization state distribution with respect to the optical axis 10 (or a generalized CV beam) to a beam having a uniform polarization state distribution. to enable. In addition, the polarization state of the beam is maintained while passing through the polarization converter 7 and only its spatial distribution is changed.

  The polarization converter system of the present invention is therefore a bijective converter with a spatial polarization state distribution. In one direction of propagation, the transducer 7 allows switching from a polarization state having a uniform spatial distribution to a cylindrically symmetric spatial distribution of the same polarization state. In the opposite propagation direction, the transducer 7 allows switching from a beam having a polarization state whose spatial distribution is cylindrically symmetric to a beam having a spatially uniform distribution of the same polarization state. .

  The system of the present invention thus makes it possible to change the spatial distribution of the polarization state independently of the polarization state of the incident beam.

  The system of the present invention allows conversion from a beam with some polarization state and a uniform polarization state distribution to a beam with a cylindrical symmetry distribution of the same polarization state.

  The polarization converter system of the present invention does not operate only in the direction of propagation for conversion from a beam having a uniform polarization distribution in Cartesian coordinates to a beam having a cylindrically symmetric polarization state distribution.

  The system of the present invention has the same polarization regardless of the polarization state of an incident beam having a cylindrically symmetric spatial distribution from a beam having a cylindrically symmetric polarization state distribution in the other light propagation direction. It offers the advantage of also allowing the conversion of the state into a beam with a uniform distribution in a Cartesian system.

  It is therefore possible to use the same transducer 7 in two directions of propagation.

  The polarization converter system of the present invention thus makes it possible to generate a beam with a polarization state distribution that is not limited to a linear or azimuth distribution of linear polarization states. On the contrary, this system makes it possible to generate any linear combination of polarization states including non-zero radial components and non-zero azimuth components. When the delay between radial and azimuth components is zero, the resulting polarization state remains linearly polarized, its spatial distribution can be cylindrically symmetric, and the polarization axis orientation is radial Neither is it a normal ray, but it is inclined at a constant angle with respect to the radial direction regardless of the local position in the beam. When the delay between the radial and azimuth components is equal to π / 2, the resulting polarization state is a circular polarization state (its spatial distribution is both uniform and cylindrically symmetric). When the delay between the radial and azimuth components is greater than zero and less than π / 2, the resulting polarization state is an elliptical polarization state, whose spatial distribution is cylindrically symmetric and the ellipse axis is It is inclined at a constant angle with respect to the local radial direction regardless of the local position.

  The ability of the system of the present invention to operate in two directions of propagation for any polarization state allows for very interesting applications.

  For example, in reflection (or transmission) microscopy applications, the return path allows the inverse transformation from polar coordinates to Cartesian coordinates. Thus, in such applications, the double pass of this system allows to compensate for the phase shift introduced only by the radial converter in both the forward and return paths, and make the system achromatic. .

  The system of the present invention therefore provides a total aperture ratio N.I. A. While having a well-controlled polarization state of cylindrical symmetry through 1.22λ / N. A. It is possible to make an imaging device (for example, a microscope) that maintains a lateral resolution of.

  Finally, no matter what the polarization state of the light beam passing through this new component 7 is, there is a comprehensive conversion or bijection between the Cartesian basis of the polarization state distribution and the polar basis of the polarization state distribution. Is produced.

  In the intended application, preferably spatially homogeneous polarized light is used so that the transformation can be characterized.

  As stated above, the use of a spatially inhomogeneous and cylindrically symmetric polarized light beam in an optical system (lens, microscope objective) whose properties are rotationally symmetric, Emphasized that it is important for some applications. Accordingly, it is contemplated that components such as those described above that are coupled to such optical systems with or without a field or aperture stop, more generally with or without a spatial or spectral filter, are contemplated. Is natural.

  FIG. 6 shows another advantageous embodiment of the invention. More particularly, FIG. 6 shows a microscope objective modified to incorporate a polarization converter system 7 according to the present invention. This objective includes a lens optical system 4 and a mechanical fixture so that it can be attached in place of a standard microscope objective.

  More generally, the focus polarization converter system comprises a Cartesian-cylindrical polarization converter 7 coupled with an optical lens, which is in front of or behind the polarization converter, ie the cylindrical system side of the converter 7. Placed in. This optical system makes it possible to manipulate the bijection between the polarization state distribution at the Cartesian basis and the polarization state distribution at the polar basis (in the more general case, spatially inhomogeneous).

  A detailed example of a Cartesian-cylindrical converter according to a specific embodiment will now be described. An example of this particular embodiment is based on the use of the radial and azimuth polarization converter component 3 provided by the Aroptix Company, the principle of which is the Staller and It is mentioned in the announcement of Schadt. It has been found that the optical path difference between the wave converted to radial polarization and the wave converted to azimuth polarization is close to 3285 nm. This optical path difference is uniform across the components. A quartz birefringent plate 6 is disposed in front of the entire radial and azimuth transducer. The thickness D of the quartz plate is selected so that its phase difference is also 3285 nm. The difference Δn between the normal and extraordinary refractive indices of quartz is generally Δn = 0.00925, and D is chosen to be D × Δn = 3285 nm, thereby giving D = 355 nm. This wave plate is arranged on the side of the Cartesian surface of the radial converter and is oriented so that the slow axis of the wave plate coincides with the fast axis of the Cartesian surface of the radial / azimuth converter. Therefore, the phase difference between the two components is compensated for each other.

The solid curve shown in FIG. 7 corresponds to a measurement of the phase shift χ between the wave converted to radial polarization and the wave converted to azimuth polarization by the components of Aroptix, It is shown as a function of wavelength λ. The amount represented is cosχ. The phase difference δ is estimated as 3285 nm by applying the following formula:
χ = 2 × π × δ / π

  FIG. 7 also corresponds to the curve, cos (2 × π × D × Δn / λ), shown as a function of wavelength, associated with the phase shift χ ′ due to the birefringence of the 355 μm thick quartz plate 6. A dashed curve is also shown.

  FIG. 8 shows the quantity χ−2 × π × D × Δn / λ as a function of wavelength. This curve is related to the phase shift between the wave converted to radial polarization and the wave converted to azimuth polarization by assembling according to the particular embodiment of the invention presented here. It has been observed that this angle, expressed in radians, remains low over a wide spectral range, which means that the phase shift between waves converted to radial and azimuth polarization is practically zero. It is made clear. Subsequently, from this characteristic, the polarization converter 7 according to this embodiment of the invention changes the spatial distribution of any polarization state to switch from a spatially uniform distribution to a cylindrically symmetric distribution, and It can be established that it is possible to do the reverse without changing the polarization state of the beam.

  The contemplated application involves optical systems that require properties with rotational symmetry that make it difficult to completely control the polarization. The use of the Cartesian-to-cylindrical polarization converter system of the present invention coupled with an optical fiber makes it possible to maintain the polarization state at the fiber input and avoids the use of polarization maintaining optical fibers.

The use of a Cartesian to cylindrical polarization converter in microscopy thus allows:
-Obtaining improved image contrast in an interference contrast microscope.
-Making observations at the Brewster angle more accessible. In fact, existing Brewster angle microscopes have an illumination arm and a focusing arm that are physically separated. The use of this type of component will allow for a more compact design, better images (no depth of field issues), and higher visibility.
At the output of the objective, for example to select the projection ring or angle.

  The cylindrical polarization converter of the present invention makes it possible to manipulate bijection between the distribution of polarization states expressed in Cartesian basis and the distribution of polarization states with rotational symmetry expressed in polar basis. To do.

  More specifically, the polarization converter of the present invention changes the spatial distribution of any polarization state to switch from a spatially uniform distribution to a cylindrically symmetric distribution and vice versa. Without changing the polarization state. Therefore, whatever the polarization state of the beam is, i.e. straight, circle or ellipse, and how the transverse direction of this polarization state is oriented with respect to the natural axis of the transducer However, it is possible to obtain a spatial polarization state distribution converter with polarization state preservation that operates in both directions.

  The present invention finds application in all fields where the use of a polarized beam is desired in an optical medium having cylindrical symmetry. In particular, the present invention is aimed at applications such as polarization microscopy, micro ellipsometry, microscopic resolution ellipsometry, micro ellipsometry imaging, and the like.

  The invention also applies to other fields, such as optical fibers that are cylindrically symmetric, for example, combining a polarized light beam with a generalized cylindrically symmetric polarization state distribution into the optical fiber. .

3 Radial polarization converter, radial converter, radial and azimuth converter, radial and azimuth polarization converter component 3a surface, input surface, Cartesian surface 3b surface, rotationally symmetric surface 4 lens, focus lens, lens optical system 5 sample 6 Wave plate, plate, quartz birefringence plate, quartz plate 7 optical polarization converter system, converter, polarization converter, polarization converter system, Cartesian-cylindrical polarization converter 8 uniform polarization beam, beam 9 beam 10 optical axis

Claims (10)

Bidirectional Cartesian-cylindrical spatial polarization state distribution transformation capable of converting a beam with a uniform spatial polarization state distribution into a beam with a generalized cylindrically symmetric spatial polarization state distribution in a first direction of propagation A Cartesian-to-cylinder converter (7) for converting a beam having a generalized cylindrically symmetric spatial polarization state distribution into a uniform spatial polarization state distribution in a second direction of propagation. The Cartesian-to-cylinder converter (7)
A radial and azimuth polarization converter (3) having an axis of symmetry (10), receiving a linearly polarized beam with a spatially uniform polarization distribution and converting it into a beam with a radial or azimuth symmetric polarization distribution; A radial and azimuth polarization converter (3) capable of conversion;
An optical means (6) for compensating for the phase difference caused by the radial and azimuth polarization converter (3), the magnitude of the phase shift χ introduced by the radial and azimuth polarization converter (3); Optical compensation means (6) capable of introducing a spatially uniform phase shift of magnitude equal to -χ so as to compensate for
A Cartesian-cylindrical spatial polarization state distribution converter (7).   The radial and azimuth polarization converter (3) comprises a first Cartesian symmetry plane (3a) and a second cylindrical symmetry plane (3b), and the optical compensation means (6 ) Is arranged on the Cartesian surface (3a) side of the radial and azimuth polarization converter (3), and the natural axis of the optical compensation means (6) is aligned with respect to the natural axis of the Cartesian surface (3a). The Cartesian-cylindrical spatial polarization state distribution converter (7) according to claim 1, characterized by:   The radial and azimuth polarization converter (3) comprises a liquid crystal converter having a first surface (3a) with liquid crystal alignment and a second surface (3b) with concentric liquid crystal alignment. Cartesian to cylindrical polarization converter (7) according to claim 1 or 2, characterized in that   It is possible that the optical compensation means (6) comprises a birefringent plate with two eigen axes, the birefringent plate introducing a phase difference between linearly polarized light oriented according to its eigen axes. The birefringent plate is arranged on the first Cartesian surface (3a) side of the radial and azimuth converter (3), according to any one of claims 1 to 3. Cartesian to cylindrical polarization converter (7).   The birefringent plate has a thickness capable of compensating for the phase difference of the radial and azimuth polarization converters over the wavelength range such that the Cartesian-cylindrical polarization converter is achromatic over the wavelength range. Cartesian to cylindrical polarization converter (7) according to claim 4, characterized in that.   The Cartesian-cylindrical polarization converter (7) according to any one of claims 1 to 5, characterized in that the optical compensation means (6) comprises a liquid crystal cell.   An optical system comprising a Cartesian-cylindrical polarization converter (7) according to any of the preceding claims, characterized in that it comprises at least one polarization beam splitting filter.   An optical system comprising a Cartesian-cylindrical polarization converter (7) according to any of claims 1 to 6, centered on the axis of symmetry (10) of the radial polarization converter (3). An optical system comprising: at least one optical lens (4), wherein the lens (4) is disposed on the cylindrical surface (3b) side of the radial converter (3).   A Cartesian-cylinder polarization state converter microscope objective comprising the Cartesian-cylinder polarization converter according to any of claims 1 to 6 and a mechanical fixture adapted to be fixed on the microscope.   A radial and azimuth polarization converter (3), and optical means (6) for compensating for the phase difference between the radial and azimuth polarization caused by said radial and azimuth polarization converter (3), comprising: Optical compensation means capable of introducing a spatially uniform phase shift of a magnitude equal to -χ so as to compensate for the magnitude of the phase shift χ introduced by the radial and azimuth polarization converter (3) (6) is used to convert a beam having a uniform spatial polarization state distribution into a beam having a cylindrically symmetric spatial distribution of the same polarization state in the first direction of propagation and / or Transforming a beam having a generalized cylindrically symmetric spatial polarization state distribution into a beam having a uniform spatial distribution with the same polarization state in a second orientation; To-cylinder polarization conversion method.

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